I doubt that you can achieve a better performance than by using the open source drivers and vgaswitcheroo (like lorenzo-s said). Sadly the 4820TG's graphics card still comes with a MUX that is not supported by the proprietary AMD drivers. The last driver that worked was fglrx 11.8 which was developed for 10.04. For all newer versions, switching with MUX was disabled and it is not likely that AMD will ever come up with a driver for that again.